The present invention relates to a communication device for an audio-video-communication (AVC) system (hereinafter referred to as an AVC communication device), and more particularly to an AVC communication device to be connected with each of individual AVC devices constituting an AVC communication system. The AVC communication system is composed of a plurality of AVC devices, e.g. an audio amplifier, television tuner, a video deck (VTR), a television monitor, a video disk player (VD), a compact disk player (CD), and a personal computer communication terminal (PC).
In a conventional AVC system, an AVC communication device, which serves to control the corresponding AVC device in accordance with a command in the communication telegram received from a communication medium, and inform the other AVC device of the communication telegram indicative of the operation status thereof, is required to recognize the communication telegram from each other and so to follow a common command code and control sequence (the protocol in an application layer); thus, in view of future extension, a command extension code is defined at a portion of a command table as shown in FIG. 9. Although such a measure in the conventional AVC system for the future extension has a possibility of adding a new control command, it in conveniently fixes the control sequence i.e. the protocol in an application layer. Further, if an AVC device which cannot sufficiently show its function in a predetermined command format appears, it is required to make a new command format. The prior art technique to solve such a problem is to define an extension command of shifting the application protocol to a new one using the extension code in the predetermined command format and, after executing this extension command, to follow the new application protocol having an entirely new command format. This prior art will be explained in detail with reference to FIG. 10.
FIG. 10 shows an AVC system comprising the conventional AVC communication devices. In FIG. 10, 111 is a VTR, 112 is a television tuner and 113 is a television monitor. These AVC devices, which are connected with the corresponding communication devices 114, 115 and 116, respectively, can send a control command and a status signal from each other through a communication transmission path 126. Communication device 114 is composed of a communication unit 117, a receiver 120 and a driver 121. Communication device 115 (or 116) is also composed of a communication control unit 118 (or 119), a receiver 122 (or 124) and a driver 123 (or 125).
FIG. 9 shows the command format to be executed by the communication control unit in the conventional AVC communication device. Each of the codes constituting a command is composed of one-byte operation code (OPC) and a plural-byte operand (OPR) qualifying OPC (as the case may be, OPC is not qualified by OPR). In FIG. 9, 101 is an area where the high order four bits are 8 to F (HEX notation) and several kinds of OPC's are defined. 102 is an area where the high order four bits are 0 to 7 and several kinds of OPR's are defined. 103 is a region which is not still defined in the OPC area, i.e. a preliminary region. 104 is a region where OPC for a general command is defined. 105 is a region where OPC for a function group command is defined. 106 is a region where OPC for a function specific command is defined. 107 is a region which is not still defined in the OPR area, i.e. a preliminary region. 108 is a region where the ASCII code is defined, or the control value for indicating or controlling increase/decrease and size are defined. 109 is a region where the standard operand indicative of `ON` and `OFF` of power supply control. 110 is a region where the code indicative of command extension at `BF` (HEX notation) in the general command is defined. The rule therefor is to add a new command code after `BF` if the command which is not still defined is required. The necessity of command extension can be given in accordance with a manufacturer's individual thought. In this case, `BF` may be succeeded by a manufacturer's code after which a new command code is added. Further, if a new AVC device cannot sufficiently show its function in the command format as shown in FIG. 9 even if the extension command (`BF`) is used, it is necessary to use another command format and application protocol. To this end, it is possible to define, in the command format of FIG. 9, a command (hereinafter referred to protocol switching command) requiring the application protocol hitherto used into a new application protocol.
Now it is assumed that in FIG. 10, service from VTR 111 to television tuner 112 is started in accordance with a new application protocol. First, communication device 114 associated with VTR 111 must require to communication device 115 associated with television tuner 112 to enter the communication mode according to a new application protocol by sending a protocol switching command to communication device 115 in accordance with the application protocol hitherto used. Until this procedure has been completed, it is not possible to execute the communication between VTR 111 and television tuner 112 in accordance with the new application protocol. However, now if television monitor 113 requires service of television tuner 112 using the command hitherto used, communication control unit 118 of communication device 115 associated with television tuner 112 cannot decode the data sent. In order to avoid this difficulty, the portion of processing an application protocol in each of the communication units must search a communication partner to ascertain which application protocol the partner uses and thereafter decode the command and data. However, the portion of processing an application protocol should essentially be dedicated to processing the application protocol but not be used for the confirmation of the kind of the application protocol. If this rule is not observed, function sharing in processings within the communication control unit cannot be clarified; thus, when a difficulty occurs, it is difficult to decide the corresponding portion.
Further, the communication frame flowing through a communication transmission path cannot usually carry a limited number of data at a time. The home bus system, which is defined in Standards of Electronic Industries Association of Japan as a system using coaxial cable and a twisted-pair cable installed with a home, actually used at present carry up to 256 bytes in one frame, and the D2B system, which is defined in International Standard as audio, video and audio-visual systems using Domestic Digital Bus (D2B) for interconnecting devices and exchanging messages needed in the audio-video cluster (TV set, VCR, etc.), can carry up to 2 (two) bytes in mode 0, up to 32 bytes in mode 1 and up to 128 bytes in mode 2. Therefore, an improved protocol to be developed in the new application protocol is always limited by the number of data which can be transmitted at a time. Namely, the application protocol cannot be taken independently of the communication system of a communication frame actually flowing through the communication transmission path.
As described above, the conventional AVC communication device has an extension command permitting command extension but it cannot take the procedure of transmitting/receiving a command, i.e. application protocol. Therefore, in the case where a new application using a new AVC device with development of technology is to be taken, it is possible to improve the function of a AVC communication system only within the range of the predetermined application protocol. Further, even if shift to the new application can be made by defining in the extension command a command which serves as a switch of switching the application protocol hitherto used into a new application protocol, software or hardware of processing the application protocols must decide which application protocol each of communication parties is to use or is using. Thus, function sharing in each software or hardware cannot be clarified so that it is difficult to decide where malfunction has occurred. Further, the home bus system or the D2B system can carry, in one communication frame flowing on the communication transmission path, only a limited number of data up to 256 bytes, 2 bytes or 32 bytes. Thus, one message cannot develop such an application protocol as does not exceed a transmittable frame length and also must consider the rule on a communication transmission path in an application.